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Characterization of new primary standards for measuring air kerma at Co-60, Cs-137 and Ir-192 gamma radiation

23.12.2021

For the primary realization of air kerma, which describes energy transmission to the medium of air via photon radiation, PTB uses specific experimental setups. In such setups, the air kerma is primarily realized by means of air‑filled ionization chambers and a number of nuclides commonly used in the medical field (Co‑60, Cs‑137, Ir‑192). The air kerma per time (air kerma rate) that is emitted by a nuclide decreases over time, resulting in a significantly low measurement signal and, thus, high uncertainties. This especially applies to low‑volume ionization chambers, like the ones currently used for this type of measurement (0.5 cm3 to 6 cm3). In a new research project, two new high‑volume cavity ionization chambers (10 cm3 and 50 cm3) were characterized for use as primary standards.

Two air‑filled ionization chambers, PS‑10 and PS‑50, were computer‑modeled in a complex procedure. For this purpose, PTB cooperated with the manufacturer of the ionization chambers (PTW, Freiburg, Germany) in order to achieve a maximum level of detail. Afterwards, various radiation transport calculations were performed to determine the radiation detectors’ behavior within the applied radiation fields (Co‑60, Cs‑137, Ir‑192). Among other things, these calculations take into account the change of the radiation field to be measured due to introducing the detector. Particular emphasis is also placed on the correct simulation of the electric field in the ionization chamber (see Figure 1). The results calculated for the respective radiation detector were summarized as correction factors. In order to validate the calculated correction factors, a number of experiments were subsequently performed, during which the air kerma rates measured with the new ionization chambers were compared to the previous primary standards.

To some extent, the calculated correction factors for the new ionization chambers differ considerably from the correction factors of the ionization chambers currently used for the primary realization of the air kerma. However, the deviations from the current primary standard for all nuclides (Co‑60, Cs‑137, Ir‑192) could be reduced to below 1 %.

The complete characterization of the new ionization chambers for use as primary standards has now been finalized.

model of the ionization chamber PS-10

Figure 1:  Geometrical model of the ionization chamber PS‑10, which was used for radiation transport calculations. The red partial volume describes the dead volume, in which no released charge carriers can be detected. This volume was determined by means of a simulation of the electric field lines within the ionization chamber.

Contact

Opens local program for sending emailStefan Pojtinger, Department 6.2, Working Group 6.25